Introduction: Under $10 Solar Box Cooker
This design is for a solar box cooker that costs less than $10. This design can cook about 3 liters of soup or 2 loaves of bread.
The maximum temperature I have had my oven at when empty so far was 163C or 326F. When cooking bread in good sun conditions the temperature is around 140C. This design has 4 inches of insulation. A pot of stew will only loose 4 degrees per hour at 85C with no sun. In another instructable I will be continuing to improve the design so it can cook faster.
It works best in places close to the equator. If you live further north or south a different mirror configuration is needed.
Step 1: Building and Painting the Oven Box
First you must determine what you would like to cook in your solar oven. I sized my oven based on a pot I wanted to use and 2 bread pans.
My oven is a 11x11 inch square that is 8.5 inches tall. I built the frame out of pine 1x2s. I build legs so the bottom surface was 4 inches off the ground (I decided to insulate the entire oven with 4 inches of insulation).
I then used inexpensive 1/8 fiber board to create the sides and bottom. I spent some time looking for a flat black, high heat, non toxic and water resistant paint. On the internet I found a charcoal powder paint. It is made using equal parts charcoal power, white glue and water. I covered a small sample of wood and placed on a pop can in a pot with boiling water. I boiled it for a few hours to check if the paint could handle the high heat and humid in this boiling environment. The paint did very well. I made a large batch of the paint and used a large paint brush to paint all inside surfaces of the oven. I used 3 coats of the paint to ensure a nice strong black color and that there is enough to protect the fiber board from humidity.
After the paint was dry I stuck my head into the oven to look for air leaks. I marked all the spots where light came through. I then used some high temperature silicon to seal and the cracks. The oven needs to be air tight so that hot air doesn't escape.
Step 2: Insulation and Insulation Box
I bought some inexpensive fabric stuffing in order to insulate the solar oven. I built a box that was 4 inches taller and was 4 inches larger in each direction. The box was made out of 1/8 inch fiber board. I joined it by screwing the sides into 1x2 blocks in the corners. I glued 2 pieces of 1/8 inch fiberboard together to form a stronger bottom piece.
Insulation was cut and placed on the bottom of the insulation box. Holes were cut into the insulation to make room for the oven legs. The oven was rapped in insulation. The oven was then carefully place in the center of the insulation box.
Step 3: Top Pieces Between the Oven and Insulation Box
Four pieces of 1/8 fiber board was used to seal the top between the oven and insulation box.
The edges were cut at 45 degrees. 4 Small 2x1 blocks were glued to the oven and insulation box so each top piece could be screwed down.
Step 4: Mirrors
First I cut 4 11x11 inch squares out of the fiberboard. I covered them in wood glue and then aluminum foil. 1 held 2 mirror together with the aluminum facing each other using clothes pins. This prevented the mirror from warping with the glue. Hinges were made using cotton fabric and wood glue. They were attached to the mirror and the top pieces. Two hinges were made for each mirror. A wood block and small piece of 1/8 ply wood was glues to the top surface. A long piece of 1/8 plywood was then secured with a clothes pin to adjust the angle of the mirror. 3 corner mirror were also built. These mirror had fabric arms that were attached with clothes pin to the square mirrors.
For the glass I used a piece of glass I found in the garbage and cut it with a glass cutter. Using double paned glass would help insulate the oven further.
Step 5: Cooking
After a few tests additional newspaper insulation was added to the top of the insulation.
This solar oven is designed to work best when the sun is directly overhead. In this situation all mirrors should be at about 60 degrees. If the sun is not overhead the mirror(s) in the direction of the sun should be at a lower angle and the mirror across form the sun should be at a larger angle. For use in areas that are far away from the equator a mirror should be built that only mounts to one side of the unit. These mirrors will have a steep angle and sides that will reflect the light from the low sun into the oven.
First bread was made in the oven. The oven was pre heated with 2 upside down bread pans so that the bread is raised closer to the top of the oven. The oven made it to 159C and the bread was added. The air temperature in the oven was about 140C while the bread was baking. The bread baked for one hour and got nice and golden brown using only the heat of the sun.
Next 3 liters of vegetable soup were made in the oven. I was slow cooked for several hours.
Chilli was slow cooked for 10 hours. The chilli was removed for one hour and bread was baked. The chilli was cooked while I was at work for 3 hours. When I got home the chilli was at 85C even though it was getting no sun heat for 1 hour. The chilli only lost about 4 degree Celsius per hour. We ate the chilli and bread for dinner that night.
4 pizza pockets were also cooked in the solar oven.
I started writing down all the food I was cooking in the solar oven without using any electricity or gas.
I love this solar oven and use it almost everyday.